fix(a11y): format Sparkline numbers using StatCard format and asset props

[dezzy-music]

Summary

Fixes #709.

StatCard rendered Sparkline without formatValue, causing describeTrend to fall back to String(v). For currency and count cards, screen readers announced raw unformatted numbers (e.g. 'trending up from 1234.5 to 8420') instead of locale-formatted currency/count strings matching the card's visible formatting.

Changes

• In src/components/ui/StatCard.tsx, passed a formatValue callback to Sparkline derived from the card's format, asset, and locale.
• Cleaned up unused isRTLLocale import in StatCard.tsx.
• Added unit tests in src/components/ui/StatCard.test.tsx asserting accessible name announcements for currency and count sparklines.

Verification

• Ran Jest: 25/25 tests pass in StatCard.test.tsx and 3/3 in dashboard-charts.a11y.test.tsx.
• Ran ESLint: 0 errors.

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